The present invention generally relates to brushless DC motors and to systems for controlling the energizing thereof, i.e. electronic commutator systems, such as are often utilized in disc drives for magnetic disc computer memory drives of the "Winchester" type.
A wide variety of brushless D.C. Motors is known in the art. In general, such devices employ a permanently magnetized rotor, such as constructed of a plurality of discrete permanent magnets or of an annular ring alternately magnetized to provide a plurality of poles, and a plurality of stator windings secured to the motor frame. The permanent magnets thus rotate with the rotor, and in lieu of brushes, electronic commutation is employed to switch current to the appropriate windings as the rotor rotates, to follow the magnetic poles, thus causing continued rotation. Such commutation systems thus include some form of non-contacting means for sensing the angular displacement of the rotor, i.e., optical or magnetic sensors. Current is then switched to appropriate stator windings in response to the sensors to cause rotation of the motor. The motor speed and uniformity thereof is ordinarily controlled and/or influenced by the sensor outputs and by supply voltage changes and load fluctuations.
Brushless DC motors of the printed circuit type, such as depicted in U.S. Pat. No. 4,228,384 are often utilized in disc drives. However, in signal reproducing apparatus employing disc drives, it is important to minimize ripple torque, i.e. to minimize surge currents required by the disc drive as well as to minimize the cost of the components utilized. Such disc drive systems typically require substantial surge currents while starting from standstill and lesser, but still substantial, surge currents during steady running, because of the widespread use of so-called "bang-bang" or "drive-coast" speed control systems. The bang-bang speed control system has a speed variation which is poorer than that obtained with a continuous speed control system.
Bang-bang speed control systems draw bursts of the full surge current in between "coasting" periods. This results in a speed which oscillates continuously up and down over a range of typically .+-.0.5%. Furthermore, as the power supply is continually subjected to surge currents even while the motor is running at nominal speed, these surge currents cause transient voltages in the power supply distribution system which in turn can have a deleterious effect on other disc drives, tape drives, etc., which typically operate on the same power distribution system.
In order to reduce the need for such surge currents, it is known to provide dual feed-back loops such that one loop enables on-off speed control during start-up, and a second loop provides continuous speed control while operating at or near the desired motor speed. U.S. Pat. No. 3,706,923 depicts one system for minimizing speed variations utilizing such a dual loop feedback concept. In the system there set forth, a closed-loop feedback servo circuit is enabled during the time that the motor is accelerating to its desired speed, after which speed control is obtained in an open-loop mode solely under control of a precise clock source so as to automatically vary the load angle or phase difference between the rotor pole centerline and the rotating electromagnetic field.
Another dual loop feedback system is disclosed in U.S. patent application Ser. No. 197,185, filed Oct. 15, 1980 (Lewis), which application is assigned to the same assignee as the present application. In the system there set forth, current to energize selected motor windings, as selected by an electronic commutator circuit, is controlled by a switching circuit operable in a first closed-loop to provide a pulse width on-off modulation control during initial start-up, and operable in a second closed-loop to provide for continuous substantially linear speed control during operation at or near a nominal operating speed. While such a linear speed control is said to enable accurate speed control, while reducing the amount of electrical noise and minimizing power supply surges, it does so through the use of an extensive, complicated and necessarily expensive circuit in which power switching transistors continue to be used to switch current to the respective windings at the appropriate times dictated by the rotor position sensors.